As commonly known, the biological purification of waste water essentially relates to the removal of carbon pollution which is characterized overall by its biological oxygen demand or BOD and the removal of nitrogen pollution.
The removal of carbon pollution is performed in the presence of oxygen by so-called heterotrophic bacteria, which use nutritive substances and convert them more simple materials, the energy released being used for the needs of the cells. In a large majority of cases, the organic nitrogen carried with the untreated waste water is converted to ammonia during this operation.
The removal of nitrogen pollution is performed in two stages:
Nitrification, the oxidation of ammonia to nitrites by bacteria of the Nitrosomonas genus and then to nitrates by bacteria of the Nitrobacter type, both of the autotrophic type; PA1 Denitrification during which the conversion of nitric nitrogen to gaseous nitrogen is performed by heterotrophic bacteria. PA1 Nitrification occurs in the presence of oxygen, similar to the removal of carbon pollution as mentioned above, and it involves autotrophic bacteria which themselves synthesize their growth factors whereas denitrification uses heterotrophic bacteria under anoxia, in a medium practically free of dissolved oxygen. PA1 processes using free cultures in which the purified bacterial flora is present in a sludge-like suspension called "activated sludge" or PA1 processes using fixed cultures in which the bacteria become fixed onto a solid support by means of gelatinous material based on exopolymers which they secrete and within which they have a degree of mobility. PA1 a pretreatment plant (grit removal, screening and grease removal) supplied with water at the nominal flow; PA1 a clarification plant designed to operate without the addition of reagents when it is supplied at the nominal flow after passage of the waste water to be treated through a pretreatment plant or and with the addition of reagents when it is directly supplied at the peak flow, that is several times greater than the nominal flow in the case of a storm for example; PA1 a fixed culture biological treatment plant through which the treated water passes next. This plant comprising biofilters for the removal of the BOD, the denitrification and the nitrification, normally placed in series, means enabling the biofilters to be placed in parallel when the nominal flow is exceeded and with means enabling the air flow rate to be adjusted in order to establish aerobic conditions in the anoxia zones in the event of an increase in the flow and in order to discontinue the aeration of the biofilters in the event of a decrease in the flow during a return to the nominal flow and, PA1 means for detecting a massive increase in the flow, especially in the event of storms, designed so as to trigger the following operations: PA1 removal of the BOD: 1 PA1 nitrification : 2 PA1 denitrification : 1
The bacteria may be used in numerous processes which can be grouped into two families:
Whichever process is chosen, the required conditions for each stage of the purification means that a specific apparatus must be provided for each function, either a single apparatus if a reduction in the biological oxygen demand (BOD) is sufficient, or three apparatuses in series for complete treatment.
Whatever the case, a plant for the biological purification of waste water consists of one or several apparatuses designed to treat a certain flow (called nominal flow) and from the hydraulic and purification point of view, can only tolerate limited increases in this flow (up to +20% approximately) otherwise the purification results and especially the nitrification results will be seriously impaired. In particular, such a plant cannot withstand heavy rain which temporarily increase the flow to a value several times higher than the nominal flow even at the cost of a decrease in purification which might be acceptable insofar as it is itself temporary.
In order to accommodate this increase in flow, it is necessary to use, during rainy periods, overflows called storm-overflows which return to the natural environment, without treatment, the excess flow above the chosen nominal flow, which obviously has disadvantages especially with respect to the preservation of the environment.